Question:My engine starts and idles fine, but lacks power
at high speed. What's wrong?
Answer:You most likely have one of two problems: a fuel
line restriction or an exhaust restriction.
FUEL RESTRICTION
A plugged fuel filter, crushed fuel line or clogged pickup
screen inside the fuel tank can all starve your engine for fuel.
Enough fuel may get through for the engine to start and run at
low speed, but when more fuel is needed it can't get through resulting
in loss of power at high speed. The same kind of problem can
also be caused by a weak fuel pump. But fuel pumps usually quit
altogether when they fail.
Inspect the fuel line from the tank to the engine. If you
don't see any obvious damage, try replacing the fuel filter.
If that doesn't help, blowing out the fuel line with compressed
air from the engine towards the fuel tank may help dislodge a
blockage and debris from the pickup screen. If the pickup screen
in the tank is clogged with rust and debris, the tank will have
to be removed so the screen can be replaced and the tank cleaned.
Fuel pressure can also be checked by teeing a suitable gauge
into the fuel line. If pressure is very low (refer to a manual
for the specs), the pump probably will need to be replaced.
EXHAUST RESTRICTION
As for an exhaust restriction, the easiest way to check for
this condition is to hook up a vacuum gauge to a vacuum port on
the intake manifold or throttle body. If vacuum is low and continues
to drop as the engine runs, it's telling you pressure is backing up
because of a restriction in the exhaust. The most likely culprit
is a clogged catalytic converter.
If the converter has overheated, it may have melted internally.
This would restrict the flow of gases through the converter and
create a serious backpressure problem that would cause a loss
of power at high speed. If the blockage is complete, the engine
may start then die and not run at all.
Other causes here may include a crushed exhaust pipe (a visual
inspection should find this easy enough), a double-walled pipe
that has collapsed internally, or a muffler that has become clogged
with rust (rare, because they usually blow out). If you suspect
an exhaust restriction, temporarily disconnect the head pipe from
the catalytic converter (which is no easy task because the bolts
will probably be rusted solid). If the engine now runs normally,
you have an exhaust restriction. Inspect the converter and replace
it if needed.
NOTE: If the converter is plugged, it failed because something
else caused it to overheat. Causes include misfiring spark plugs
and leaky exhaust valves. The underlying problem needs to be
identified and corrected before the converter is replaced otherwise
the new converter will suffer the same fate.
